The phrase "break into a smile" is often used to describe the expression of sudden happiness or joy on someone's face. However, there are many other ways to express this sentiment, and it can be fun to switch up your vocabulary. Some alternatives to "break into a smile" could include "burst into a grin," "crack a smile," "light up," "beam," "smirk," or "flash a smile." Each of these phrases conveys a slightly different mood, so consider the context in which you want to use them. Experimenting with synonyms can add variety and nuance to your writing or speaking.